Using certain resources on the Docker host

**Description:**

It'd be nice if the installation documentation could outline how to use certain resources on the host with the Docker container. In my particular case, it's a Postfix daemon running on the host. After having spent the better part of the last few weeks of available time trying to figure this out, I'm just about to give up.

_(In my particular situation, the actual server has a number of applications running on it, and putting a Postfix container inside each Docker instance seems like an awful waste, and a configuration nightmare.)_